Geoscience Reference
In-Depth Information
For cyclic loading and vibrations (earthquake) specific constitutive models have
been developed, suitable to simulate pore pressure built-up, meta-stability and
liquefaction.
C ASPECTS OF NUMERICAL SIMULATION
For the design of geotechnical structure, it is necessary to satisfy four basic
requirements, i.e. equilibrium (stresses), compatibility (continuity of strains),
material constitutive behaviour (stress-strain relation) and boundary/initial
conditions (displacements and/or stresses). Since soil behaviour is rather complex,
in most cases, approximations must be introduced and numerical simulations are
adopted.
A simplified numerical approach used to model soil-structure interaction
behaviour approximates the soil as a set of unconnected vertical and horizontal
springs or represents structural supports (props or anchors) by springs. This is
commonly known as the "beam-spring" approach. In the full numerical analysis
approach, attempts are made to satisfy all theoretical requirements, include realistic
soil constitutive models and boundary conditions. Approaches based on finite
difference, boundary element and finite element methods are those most widely
used and sometimes analytical element methods are applied. Their ability to
accurately reflect field conditions depends on the suitability of the constitutive
model to represent real soil behaviour and boundary conditions in all design stages.
In this manner, the full numerical approach can contribute to efficient application
of the observational method. The power of numerical analysis is that in simulations
the entire geometry can be considered without trivial assumptions such as adopted
in the slip circle analysis. For elasto-plastic soil behaviour, the code finds self a
failure state and corresponding deformations in the surrounding. The abundant data
output has to be visualised in suitable forms to provide comprehensive insight.
Numerical analysis allows evaluating the typical non-linear soil behaviour. Full
numerical analyses are however complex and specific experience is required for a
proper use of advanced numerical codes.
Such experience is related to spatial and time discretisation, such as element
shape, size, order and properties and dimensions (3D to 2D), and numerical
aspects, such as non-linear iterative techniques and convergence criteria. For
nonlinear constitutive soil models, a reasonable stress distribution within the
element and a consistent initial state are essential. Higher order elements should be
used (at least quadratic elements for elasto-plastic analysis). To improve accuracy,
the discretisation should be fine at the edge of a footing or in a retaining wall and
in regions where the stress and strain changes and porous flow changes are high. In
transition zones and soil-structure interfaces, compatibility is properly maintained
by adopting special elements with suitable interpolating functions.
The quality of calculation results depends on the stability and accuracy of the
algorithms and on the constitutive model and corresponding parameter selection.
Some of these parameters may have significant influence on the results. Depending
on the type of soil and precision required, constitutive parameters can be
determined from pressuremeter tests, dilatometer tests, CPT (lower reliability,
especially for clays), SPT (lower reliability), in-situ elastic wave velocities (very
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